Doppler ultrasound is widely used to measure blood flow velocity in clinical applications. In a standard Doppler ultrasound blood flow measurement, an experienced doctor first selects an appropriately sized cuff and places it around the limb at a location above the artery of interest. The doctor needs to wait for the Doppler sound to become stable and rhythmic after the Doppler probe has been well placed over the artery. After that, the doctor inflates the cuff to gradually increase the cuff pressure till the Doppler sound disappears, which implies that the artery is already fully occluded. Important information is provided by the moment at which the status of the artery changes from open to closed (or vice versa) because this moment is very crucial for determining some parameters in the blood flow measurement. For example, the corresponding pressure read from the sphygmomanometer at the moment immediately before this switch moment (at which the status of the artery changes from open to closed) is defined as the inflation systolic blood pressure (SBP). The inflation SBP may be correctly measured if the open to closed switch moment is correctly determined.
Then, the doctor further inflates the cuff until the cuff pressure is 20 mmHg to 30 mm Hg over the inflation SBP, after which he slowly deflates the cuff until the Doppler sound reappears. Similarly, the corresponding pressure read from the sphygmomanometer at the moment when the doctor hears the first Doppler sound in the deflation process is defined as the deflation SBP, physiologically describing that the status of the artery changes from closed to reopened. Doppler ultrasound is used as the gold standard to determine the open/closed switch moment of an artery of interest and furthermore to measure the systolic pressure.
However, the automatic determining of the open/closed switch moment of an artery of interest under a changing pressure is only based on the change of a continuous ultrasonic signal. How to correctly detect the last sound in the inflation phase and the first sound in the deflation phase is a fundamental problem.
Conventionally, it could be mathematically summarized as a feature mapping problem. Inspired from the standard blood flow measurement, three typical parameters of blood flow, i.e. sufficient power of the sound, reasonable velocity of the blood flow and stable rhythm corresponding to the heart beat, are the straightforward features to map the open status of the artery. As long as one of the features has a sudden change during cuff inflation, i.e. outside of the feature threshold, the artery is deemed to be closed. For the cuff deflation phase, it is necessary to request a safe condition of first sound recognition, i.e. that all the features of the Doppler sound should be restored when the status of the artery changes from closed to reopened.
However, these three features mentioned above are not easily employed by a variety of the users, especially with respect to finding the proper thresholds to discriminate the two statuses of the artery for the patient with artery disease. All these features may change from person to person, even for the same person at different times, which causes the problem of false determination of the open/closed switch moment.
For example, the problem of using the power feature is that: because the distal stenosis for the patient with atherosclerosis is ischemic, the Doppler sound presents a low power property when the transducer is placed at the distal stenosis. However, for the same patient the power situation at the proximal stenosis differs from that at the distal stenosis. The power of sound is as high as that of the normal artery due to the power contribution from the turbulence and laminar flow around the site of stenosis. The amplitude of the sound power depends on the grade of artery stenosis, so that those skilled in the art cannot set one power threshold for all measurement cases. Furthermore, low power creates an unapparent power boundary between the open and closed status of the artery, making it difficulty to set a valid power threshold. The situation with respect to the patient with artery calcification is similar to the low power situation. It is usually hard to find a steady duration in which the power is obviously low, because the artery of a patient with artery calcification cannot be fully occluded.
Further, the problem of using the velocity of the blood flow may be as follows: a few factors may affect the measured velocity of the blood flow. With regard to hemodynamics, these factors include: diameter of the artery, consistency of the blood and stiffness of the artery wall. The hemodynamic parameters will be deformed by the state of illness of the patient. With respect to operational work, the main factor that influences the observed value of the velocity of the blood flow is the transmission angle of the ultrasound. Because of the variability of the blood flow velocity, the velocity threshold to discriminate the status of the artery should be customized for different users, which makes the algorithm very complicated.
Furthermore, the problem of using the sound periodicity is that: the subject's heart rate is highly related to the periodicity of the Doppler signal. A person's heart beat is sensitive to his emotions and stimuli from external circumstances. It is really common that a person's heart rate accelerates from 80 bpm to 120 bpm when he feels nervous or uncomfortable. This may also happen during the blood flow measurement. Although the subject's heart beat can be estimated in the calibration stage, the heart rate of normal persons will likely vary a lot as the cuff pressure increases or decreases, quite unlike a patient with arrhythmia. The degree of rhythmic alteration is unpredictable beforehand, so that it is really hard to set the periodicity threshold well in the case of people who easily feel nervous or distressed. If the rhythmic alteration is underestimated or overestimated, the estimated switch moment will correspondingly have a positive or negative lag.
It can be seen from the above that an important reason why the above-mentioned problems cannot easily be solved is that it is impossible to describe the behavior of the subject's blood flow without any calibration procedure as well as to adapt the algorithm for the change during the measurement using the calibration forecast.
The conventional method is not usable to determine the precise switch moment for the patient with artery calcification, stenosis and arrhythmia. If a method focuses on the change of the power of the Doppler sound, the method has an error risk because the power change may be extremely small in the case of the patient with the artery calcification or stenosis. If a method focuses on the change of the period of the Doppler sound, the method cannot be effective in the cases where the subject is a patient with arrhythmia or a person whose heart rate is easily disturbed.
In view of the above, there is a need for a method which adopts more effective and adaptive features so as to make the determination of the switch moment more reliable, and which method is adaptable to a wide range of uses, including even patients with artery diseases, such as arrhythmia, artery stenosis and calcification.